Optical link monitoring solutions use current interfaces and current-to-voltage conversion with a resistor followed by an analog to digital converter (ADC). FIG. 1 illustrates such a related art system 50 that includes a receiver 55 (e.g., optical receiver that senses light using a photo detector), an ADC 60, and a resistor R1. The resistor R1 converts a monitoring current Im into a monitoring voltage Vm. The ADC 60 converts the voltage Vm into a digital signal that is used (e.g., by a digital signal processor (DSP)) for further processing. One disadvantage of this approach is that for a high optical dynamic range (e.g., greater than 30 dB) of the receiver 55, the voltage range prior to the ADC 60 is even higher (e.g., greater than 1000:1 or 60 dB) for the current, or a factor of two in dB, compared to optical power due to the quadratic characteristic of the photo detector. Furthermore, due to linearly sized steps of the ADC 60, low voltages are sampled too coarsely while high voltages (for high optical power) are sampled too finely. Thus, to accurately measure the low monitoring currents, a large overhead of bits of the ADC 60 is used or the ADC 60 employs special range switching features, both of which increase cost.
The low current detected by the receiver 55 can be on the order of 1 uA or even lower for a high dynamic range receiver if the upper limit is fixed or limited to low power applications. However, low cost ADCs, such as those implemented in related art systems, usually have high leakage current (e.g., on the same order as the lowest measurable current of the receiver 55) and additionally have a limited number of bits (e.g., about 10-12 bits or 1024-4096 steps) for the conversion, which makes them unsuitable for low cost, high dynamic range applications. Further, often more than one parameter (besides current) is to be measured (e.g., average power, modulation power, etc.), which further complicates the system by adding multiple ADCs or multiplexers, which also have high leakage current in the low cost range. An additional disadvantage is the sensitivity to electromagnetic interference (EMI), since for low currents the voltage can go below 1 mV.
Another approach is to measure the current by using a low cost processor or microcontroller, but this approach has computational costs and associated inaccuracies of the measurement. This is because microcontroller based measurement is done with help of software or hardware interrupts. Edges of a signal being monitored are detected with interrupts and a timer is used to calculate time taken between edges. The interrupt and timer based traditional approach has following disadvantages, especially on 8 bit microcontroller with 16 bit timers: 1) interrupts are an overhead as for every interrupt normal program flow switches to interrupt execution—thus, regular incoming interrupts may affect other protocols handled by microcontroller measuring the frequency; 2) time recording done in interrupt may have latencies if a high priority interrupt is already in processing, which leads to inaccurate time stamp recording; and 3) 16 bit timer registers overrun for lower frequencies as the timer is configured with microsecond resolution. If timer resolution is reduced, frequency measurement accuracy is severely compromised, especially in the high frequency range. Timer overflow handling may be achieved with timer overflow interrupts, which causes even more system load affecting time critical protocols running on the microcontroller.